Electrophotographic printers of a type using a laser beam line-scanning hitherto employ, as the laser beam, a gas laser of relatively short wavelength such as helium-cadmium laser, argon laser, helium-neon laser, etc., and as an electrophotographic photoreceptor therefor, CdS-binder type photosensitive layer and a charge transfer complex which are capable of forming a thick photosensitive layer (IBM Journal of the Research and Development, 1971, January, pp. 75-89). In such electrophotographic printers, no multiple reflection of the laser beam occurs within the photosensitive layer; and interference fringes have not been encountered practically at the image formation.
However, semiconductor lasers have come to be recently used in place of the gas lasers for the purpose of miniaturization and cost reduction of the apparatuses. Such semiconductor lasers generally have oscillation wavelengths in a long wavelength region of 750 nm or longer, creating needs for an electrophotographic photoreceptor having high sensitivity in long wavelength regions; and electrophotographic photoreceptors for such purpose have been developed.
As photoreceptors sensitive to long-wavelength light (e.g., 600 nm or longer), among the typical known types are lamination type electrophotographic photoreceptors having a photosensitive layer containing a phthalocyanine pigment such as copper phthalocyanine, and aluminum chloride phthalocyanine. Specific variations include a photosensitive layer having a lamination structure comprising a charge generating layer and a charge transporting layer, and electrophotographic photoreceptors employing selenium-tellurium film.
Such a photoreceptor sensitive to long wavelength light has a disadvantage such that, when it is mounted on a laser-beam-scanning type electrophotographic printer and is exposed to a laser beam, an interference fringe pattern appears in the toner image formed thereby to cause unsatisfactory image formation. One reason for this is believed to be that the long-wavelength laser radiations are not completely absorbed in the photosensitive layer, and the transmitted light specularly reflects at the surface of the substrate, forming multiple-reflection paths of the laser beam within the photosensitive layer, and interference occurs between the incident light and the reflected light at the surface of the photosensitive layer.
For solving the above problem, methods for roughening the surface of the electroconductive substrate employed in electrophotographic photoreceptors by anodic oxidation or buffing are proposed as described in JP-A-58-162975, JP-A-60-79360, JP-A-60-112049, JP-A-61-42663 and JP-A-62-186270 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"); and also methods for eliminating multiple reflection occurring within the photosensitive layer by providing a light absorption layer or a reflection prevention layer between the photosensitive layer and the substrate are proposed as described in JP-A-58-17105, JP-A-59-158 JP-A-59-204048, and JP-A-60-86550.
The above-described proposed methods, however, could not completely eliminate the interference fringe pattern appearing in forming images practically. In particular, in roughening the surface of an electroconductive substrate, uniform roughness of the surface cannot easily be obtained, and sometimes a portion of relatively coarse roughness is formed in a certain proportion. The coarse roughness portion may function as a portion for injecting carriers into a photosensitive layer, thereby causing undesirably a white spot in image formation (or a black spot in negative development). Thus, there are various measures for preventing solely the appearance of interference fringe pattern; but simultaneous prevention of the occurrence of interference fringe patterns and prevention of the occurrence of black spots or white spots on the images is extremely difficult. Thus, the above methods cannot solve the problems of long-wavelength photoreceptors. Moreover, in the method for roughening the surface of the electroconductive substrate, production of electroconductive substrates having uniformly toughened surface through one production lot is difficult, involving many problems to be solved. On the other hand, the methods for employing a light absorption layer also have the disadvantage that they are incapable of preventing sufficiently the interference fringes and will increase the production cost.
Other superficially relevant techniques are not solutions. JP-A-51-58954 describes surface toughening of an electroconductive substrate by honing. JP-A-59-128553 describes surface roughening with a specific surface treating material. These descriptions are directed to improvement of adhesion of the photosensitive layer to the substrate but are incapable of preventing the appearance of the aforementioned interference fringe pattern.